In late May 2009, a decision was made to monitor clusters of influenza cases in order to detect the circulation of the 2009 H1N1 virus in the population. On Monday, June 22, the principal of the Jeu de Paume school in Créteil - Val-de-Marne, reported high absenteeism due to flu-like symptoms. Following the confirmation of two cases of influenza A (H1N1) 2009 and concerns from parents, the emergency medical services (Samu) set up a temporary clinic in a tent in the schoolyard on June 24 and 25. On June 26, the InVS unit in the Île-de-France region and the Departmental Directorate of Health and Social Affairs 94 launched an investigation. In total, 68 people (40 students, 2 teachers, 26 parents or relatives) presented with confirmed or probable influenza. They were identified during the 505 consultations conducted at the school site or at the hospital, during which 189 preventive treatments with oseltamivir were prescribed to contacts. In the absence of the onset date of symptoms among the cases, the dynamics of the influenza outbreak were determined by reconstructing the absenteeism rate. This rate, which was below 5% prior to the event, reached 11% on the Friday preceding the report and was around 20% on Monday the 22nd and Tuesday the 23rd before the school closure ordered by the school district. Among the confirmed or probable cases, cough was a nearly constant symptom, while fever was present in only 2 out of 3 cases, body aches in 40% of cases, and fatigue and headaches in one-quarter of cases. Two children who developed symptoms on June 18 are likely to be the index cases. Neither had traveled or been in contact with anyone who had traveled abroad. The last case attributable to the outbreak developed symptoms on June 26, three days after the school closed. The outbreak began in a single classroom at the elementary school and then spread throughout the entire school as well as to the preschool, without any risk factors for transmission being identified. However, the school choir rehearsal, which included all elementary school students on the very day the first cases developed symptoms, may have played a role in the transmission of the virus. Only 6 people, including 4 children, were tested for the virus at the start of the outbreak. Three children tested positive for the 2009 A(H1N1) virus via PCR. Subsequently, the focus shifted to the rapid management of all symptomatic individuals and their contacts. The information gathered by clinicians during urgent consultations was insufficient to accurately describe the event, despite active case and data surveillance. This cluster involving a large number of people, occurring after those in Toulouse and Paris’s 15th arrondissement, confirmed the circulation of the virus in the population. (R.A.)